Inspection system for oled display panels

ABSTRACT

A system for inspecting at least a portion of a display panel having thin film transistors (TFTs) and light emitting devicxes (OLEDs), during or immediately following fabrication, so that adjustments can be made to the fabrication procedures to avoid defects and non-uniformities. The system provides bonding pads connected to signal lines on at least portions of the display panel, and probe pads along selected edges of the display panel. The probe pads are coupled to the bonding pads through a plurality of multiplexers so that the number of probe pads is smaller than the number of bonding pads.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/861,614, filed Aug. 2, 2013, and U.S. Provisional Application No.61/814,580, filed Apr. 22, 2013, both of which are hereby incorporatedby reference herein in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to OLED displays and, moreparticularly, to inspection systems for detecting defects andnon-uniformities in displays such as active matrix organic lightemitting diode displays.

BACKGROUND

Display panels can be created from an array of light emitting deviceseach controlled by individual circuits (i.e., pixel circuits) havingtransistors for selectively controlling the circuits to be programmedwith display information and to cause the light emitting devices to emitlight according to the display information. Thin film transistors(“TFTs”) fabricated on a substrate can be incorporated into such displaypanels. Both OLEDs and TFTs can demonstrate non-uniform behavior acrossdisplay panels due to production problems. Such problems can becorrected if the defects and non-uniformities can be identified at thetime the panels are produced, e.g., during or immediately followingfabrication.

SUMMARY

A system is provided for inspecting at least a portion of a displaypanel having thin film transistors (TFTs) and light emitting devicxes(OLEDs), during or immediately following fabrication, so thatadjustments can be made to the fabrication procedures to avoid defectsand non-uniformities. The system provides bonding pads connected tosignal lines on at least portions of the display panel, and probe padsalong selected edges of the display panel. The probe pads are coupled tothe bonding pads through a plurality of multiplexers so that the numberof probe pads is smaller than the number of bonding pads.

The foregoing and additional aspects and embodiments of the presentinvention will be apparent to those of ordinary skill in the art in viewof the detailed description of various embodiments and/or aspects, whichis made with reference to the drawings, a brief description of which isprovided next.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings.

FIG. 1 is a diagrammatic perspective illustration of a display paneladapted to receive a probe card.

FIG. 2 is a diagrammatic front elevation of the display panel shown inFIG. 1, showing the locations of probe pads for receiving probe cards.

FIG. 3 is a diagram of a pair of probe pads connected to a multiplexerused to supply probe signals to the probe pads.

FIG. 4 is a schematic circuit diagram of one of the probe padsillustrated in FIG. 3 connected to receive the display signals.

FIG. 5 is a diagrammatic illustration of an inspection system for takingmeasurements of an AMOLED panel, and various corrective actions that canbe taken to fix defects identified by analysis of the measurements.

FIG. 6 is a schematic circuit diagram of a pixel circuit having a signalWR.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. Itshould be understood, however, that the invention is not intended to belimited to the particular forms disclosed. Rather, the invention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

FIG. 1 illustrates a system for inspecting an OLED display panel 10 atone or more stages of the fabrication of the panel 10 (e.g., a TFTbackplane, a fully fabricated panel, or a fully completed and sealedpanel). The display panel 10 is coupled to a computer 14 throughmeasurement electronics 11 and a probe card 12, to provide thecapability of testing and verifying the panel at each processing step.For example, after finishing the TFT backplane, the probe card systemcan be used to measure the performance of the TFT backplane by itself.If the TFT backplane is acceptable, then the panel 10 is passed to thenext step which can be the OLED deposition stage. After the OLEDdeposition is completed, the panel 10 can be measured for proper OLEDdeposition before sealing. After sealing, the panel 10 can be measuredagain before it is sent to an assembly process.

As can be seen in FIG. 2, the illustrative display panel 10 has probepads 20 formed along three of the four edges of the panel. Probe padscan also be formed inside the panel, preferably before the OLEDdeposition stage. The probe pads 20 are used to supply test signals tothe numerous pixel circuits on the display panel 10, via bonding pads 30formed at the outer ends of the various signal lines leading to thepixel circuits.

FIG. 3 illustrates the connection of the probe pads 20 to the bondingpads 30 through a multiplexer (MUX) 40, to reduce the required number ofprobe pads, which in turn permits the pad pitch to be increased. Toensure that the other signals connected to the probe pads 20 are biasedproperly, the MUX 40 needs to be capable of connecting each probe pad 20to a common signal (Vcom) for each group of signals (e.g., sourcesignals, gate signals, etc.).

FIG. 4 illustrates a MUX 40 with common signal control and two or morepanels signals for each probe pad 20. FIG. 4 illustrates h panel signalsconnected to one probe pad 20, thus requiring 2 h controlling signalsfor connection to the probe pads 20 or connection to the common signals.The connections of the panel signals to the probe pad 20 are controlledby first switches 41 and 42, and the connections of the common signalsVcom to the panel signal lines are controlled by second switches 43 and44.

The proper pad pitch for full panel probing is typically about 150 μm.As illustrated by the data in Table 1, the pad pitch for mostconventional configurations meets the minimum pad pitch requirements.However, using multiplexing ratios of 2:1 or greater permits the padpitch to be increased, resulting in much simpler probe cards, as alsoillustrated by the data in Table 1:

TABLE 1 Pad pitch for different display sizes and resolutions. Gate PadSource Pad EIC Pad Pitch Pitch Pitch Panel MUX (μm) (μm) (μm) 55″ HD 1:1295 330 330 55″ HD 2:1 592 661 661 55″ HD 8:1 2370 2645 2645 55″ UD 1:1148 165 165 55″ UD 2:1 295 330 330 55″ UD 8:1 1185 1132 1132 78″ UD 1:1222 222 222 78″ UD 2:1 444 445 445 78″ UD 8:1 1777 1781 1781

As depicted in FIG. 5, an electronic measuring system 13 mounted on theprobe card 12 can measure the electrical characteristics of every TFTand every OLED device in a display panel 10 and identify defects andnon-uniformities. This data is supplied to a GUI 14, where the data canbe used to fine-tune every process step, to achieve higher yields,faster process ramp-up, and lower line monitoring costs. Examples of thevarious process steps that can be fine tuned are illustrated in FIG. 5,namely, a sputtering and PECVD module 50, a process annealing module 51,a patterning module 52, a laser repair module 53, an inkjet printingmodule 54 and an evaporation module 55. The end result is a completedisplay panel 56.

The circuitry depicted in FIG. 5 takes data from the measurementelectronics 13, analyzes that data, and displays it in a wide variety ofreports, tables, and pictures. Some of the views are described in thefollowing table:

View Description TFT Absolute View the absolute measurement replacementvalues LUT for each pixel on the panel. TFT Filtered View the filteredreplacement values used to LUT calculate delta values. TFT Base View thefactory shipment values of the panel LUT to determine how much thepixels have aged (baseline). TFT Delta View the difference between thecurrent average LUT measured value and the baseline values (used todetermine compensation). TFT Histogram View the number of times a pixelhas been measured LUT since the last time the histogram was reset. Thislookup table is primarily used for priority scan algorithm. TFT PixelView either the current state of the measurement State LUT state machineor the last comparison values for each pixel. TFT Region Show thepriority of each region in the priority Priority scan algorithm. OLEDAbsolute View the absolute measurement replacement values LUT for eachpixel on the panel OLED layer. OLED Filtered View the filteredreplacement values used to LUT calculate delta values. OLED Base LUTView the factory shipment values of the panel to determine how much thepixels have aged (baseline). OLED Histogram View the number of times apixel has been measured since the last time the histogram was reset.OLED Pixel View either the current state of the measurement State statemachine or the last comparison values for each pixel. OLED Region Showthe priority of each region in the priority Priority scan algorithm.Dead Pixels Show which pixels were either dead at point of LUTmanufacture or have since been determined to be unresponsive. Note thatdead pixels are not compensated. Combine Delta The combined TFT and OLEDdelta values used to LUT determine the final compensation. Scratch LUT Atemporary LUT View to allow users to manipulate the data without makingmodifications to the system tables or to simply “backup” a table.Statistics Reports the performance statistics and the current framerate. Use these statistics to compare the time required to process anddisplay the current data in MaxLife Viewer vs. the time it wouldactually take if the display was not required. Pixel Trends View thepixel state to determine if there are unsettled pixels and viewcomparative levels. Allows you to obtain a visual representation ofuncertainty zones and look at specific defective pixels. Use this optionto obtain active measurements over time; determine how many times apixel was measured before it settled. Uniformity Select a LUT table,then analyze the uniformity. Report Offset Character- Plots the V-to-Ivoltage DAC code to the probability ization of a comparator result flip.Hardware View the current hardware configuration parameters.Configuration Display Adds the Display Controls to the bottom of theControls current tab. These options allow you to set the frame refreshrate in frames per second. Drag the slider to the left or right to speedup or slow down the refresh rate. Slower speeds are more visible to thenaked eye. CLI View Issues commands from MaxLife Viewer to the system.

A wide variety of different circuitry and algorithms may be used forextracting measurements of different parameters from the display panelat different stages of its fabrication, such as the extraction systemsdescribed in U.S. patent application Ser. No. 13/835,124 filed Mar. 15,2013 and entitled “Systems and Methods for Extrraction of Threshold andMobility Parameters in AMOLED Displays,” which is incorporated byreference herein in its entirety.

The inspection system can identify many potential defects and problems(e.g., with sputtering and PECVD steps, that can be used to identify thelikely cause of the defect or problem so that the fabricating processcan be immediately fine-tuned to correct the problem). Examples of suchissues and their likely causes are the following:

Issue identified by MaxLife ™ inspection system Likely cause Line defect(open circuit Particle defect during deposition, on metal lines) pooradhesion, contaminated substrate and poor step coverage. High resistanceor non- Non-uniform sputtering process, uniform resistance on metalcontaminated sputter gas or process lines chamber. Non-uniform TFTcontact Problem with n+ layer PECVD step, resistance incomplete via etchor photoresist stripping process. Vt or mobility of TFTs out Problemwith a-Si layer PECVD step of specification (contamination duringdeposition, process parameter drift or film stress) Open TFT channelProblem with a-Si or n+ layer deposi- tion (particle contaminationduring deposition/contaminated substrate) Gate shorted to fixedIncomplete metal patterning and/or voltage damaged dielectric layersfrom particles or ESD Source or gate shorted to Damaged dielectriclayers (pinholes) drain from particles or ESD Open or high resistanceIncomplete via etching. contacts Out of spec capacitance Non-uniformdielectric layer deposi- tion or drift in deposition process parameters.Line defect (crossover Pinhole in the dielectric layers from short)particles or ESD

For defects that cannot be directly identified by a single measurementof the inspection system, the first measurement can reveal that aproblem exists, and specify additional tests that will conclusivelyidentify the exact defect. One example is the identification of linedefects, which can be detected by any of the following procedures:

-   -   1. Measuring the current of different lines: if the current is        higher than a threshold, the pixel is shorted.    -   2. Applying pulse to measure the charge transfer: if the amount        of charge transfer is smaller than a threshold, the line is        open.    -   3. For a signal with connection to DC current (e.g., Vdd and        Vmonitor), the current can be measured to detect the open defect

Defects in the thin film transistors (TFTs) can also be detected. Forexample, in the situation where the pixel circuit in FIG. 6 has a signalWR measured as high (while Vdata=high, and also while Vdata=low, andVdd=high), an additional test needs to be performed. Table 1 shows thedifferent conditions and what the results mean.

To detect problems with process annealing, the exact Vt and mobility ofeach TFT can be used to adjust process annealing parameters, as follows:

Issue identified by MaxLife ™ inspection system Likely cause Vt and/ormobility of TFTs is higher Laser power drift or lower than specificationSmall scale non-uniformity of Vt Intermittent laser power and/ormobility of TFTs output Large scale non-uniformity of Vt Laserrepeatability and/or mobility of TFTs

The number and types of defects can be used to identify problems inpatterning (particles, under/over exposure, etc.), as follows:

Issue identified by MaxLife ™ inspection system Likely causeHigh-resistance metal lines Pattern definition or metal etch process.Poor line width control. Open or high resistance Poor via patterndefinition/photo- contacts resist residue Gate shorted to fixed Pinholesin the dielectric layers. voltage Abnormal capacitance or Mask alignmenterror (rotation), resistance in corners of photoresist thicknessnon-unifor- panel mity. Large scale capacitance or Pattern alignmenterror or exposure resistance, non-uniformity power fluctuation. Adjacentmetal lines shorted Particles in photoresist/pattern definition. Patternstitching defects Stepper alignment failure Repetitive defect Exposuremasks damage or contami- nated.

The defect location and defect type can be used to pinpoint areassuitable for laser repair (removing material) or ion beam deposition(adding material), as follows:

Issue identified by MaxLife ™ inspection system Repair Step Gate shortedto fixed Give exact pixel location to laser voltage repair system Shorton metal lines Identify the metal lines that are shorted. Open circuiton metal lines Identify the metal lines that are open. Open or highresistance Quickly identify the number and TFT contacts location of thedefective pixels.

The uniformity data can also be used to continuously calibrate eachprint head used for inkjet printing, in real-time. The system knowswhich print head was used to print each pixel, and thus problems withindividual print heads can be detected. The print head used to printthose pixels can then be immediately adjusted, as follows.

Issue identified by MaxLife ™ inspection system Likely cause Dead pixelsPrinthead occasionally putting down too little material, causing shortsStuck-on pixel Printhead occasionally putting down too little materialHigh-resistance pixels The printhead printing those pixels may beputting down too much material Uniformity of OLED's Flow control ofprinthead malfunc- voltage is poor tioning

The exact failure mode of every OLED device can be used to tune theevaporation process, as follows:

Issue identified by MaxLife ™ inspection system Likely cause All pixelsfrom one printhead Problem with calibration of are too high (or too low)printhead resistance Short-circuit OLED Too little organic materialbeing deposited, causing shorts High-resistance pixels Too much organicmaterial being deposited OLED voltage too high Too much organic materialbeing deposited Long-range Uniformity of Problem with substrate rotationor OLED's voltage is poor evaporator too close to substrate Short-rangeuniformity of Problem with thermal evaporation OLED's voltage is poortemperature control Open-circuit OLED Particles during evaporation Shortto cathode or anode Particles during evaporation Partial short (lowresistance) Too little organic material being deposited

The electrical characteristics (collected during TFT and OLEDinspection) can be loaded into a lookup table, and used to correct forall TFT and OLED non-uniformities.

Additional defects can be identified once both the OLEDs and TFTs havebeen deposited. The first measurement can identify that a problemexists, and specify additional tests that will conclusively identify theexact defect.

If test samples are created around the periphery of the panel, then moredetails about the global process parameters can be extracted. Typicallythis is done by cutting off the test samples from a small percentage ofdisplays and putting them in a separate characterization system.However, with the present inspection system, this can be done as part ofpanel characterization, for every panel, as follows:

-   -   Metal lines can be created and resistance measured. This can        test both metal deposition steps and etching.    -   Semiconductor layers to be annealed can have their        characteristics and uniformity tested.    -   Structures can be used at different locations around the panel        to test alignment.    -   OLED structures can be used to test evaporation and inkjet        printing steps.

While particular embodiments and applications of the present inventionhave been illustrated and described, it is to be understood that theinvention is not limited to the precise construction and compositionsdisclosed herein and that various modifications, changes, and variationscan be apparent from the foregoing descriptions without departing fromthe spirit and scope of the invention as defined in the appended claims.

1. A method of inspecting at least a portion of a display panel havingpixel circuits that include thin film transistors (TFTs) and organiclight emitting devices (OLEDs), during or immediately followingfabrication, so that adjustments can be made to the fabricationprocedures to avoid defects and non-uniformities in said pixel circuits,said method comprising providing bonding pads connected to signal lineson at least portions of the display panel, providing probe pads alongselected edges of said portions of the display panel, and coupling saidprobe pads to said bonding pads through a plurality of multiplexers sothat the number of probe pads can be smaller than the number of bondingpads.
 2. The method of claim 1 which includes measurement electronicscoupled to said probe pads, and a computer coupled to said measurementelectronics.
 3. The method of claim 1 in which said probe pads areformed along a plurality of edges of said display panel.
 4. The methodof claim 1 which includes supplying test signals to the pixel circuitson the display panel via said probe pads.
 5. The method of claim 1 inwhich said display panel is an AMOLED display panel.
 6. The method ofclaim 1 in which each of said multiplexers is capable of connecting eachprobe pad to common signals (Vcom) for multiple groups of signals. 7.The method of claim 1 which each multiplexer includes first switchesproviding connections of multiple panel signal lines to a probe pad, andsecond multiplexers providing connections of common signals to panelsignal lines.
 8. The method of claim 1 which includes measuring theelectrical characteristics of every TFT and every OLED device in thedisplay panel, and identifying defects and non-uniformities in the panelbased on the measured electrical characteristics.
 9. A display panelhaving a multiplicity of pixel circuits that include thin filmtransistors and organic light emitting devices OLEDs), said displaypanel comprising multiple signal lines on portions of the display panel,multiple bonding pads connected to said signal lines, multiple probepads positioned along selected edges of the portions of the displaypanel that have said signal lines, and multiple multiplexers couplingsaid probe pads to said bonding pads.
 10. The display panel of claim 9which includes measurement electronics coupled to said probe pads, and acomputer coupled to said measurement electronics.
 11. The display panelof claim 9 in which said probe pads are formed along a plurality ofedges of said display panel.
 12. The display panel of claim 9 which saidprobe pads supply test signals to the pixel circuits on the displaypanel.
 13. The display panel of claim 9 in which said display panel isan AMOLED display panel.
 14. The display panel of claim 9 in which eachof said multiplexers is capable of connecting each probe pad to commonsignals (Vcom) for multiple groups of signals.
 15. The display panel ofclaim 9 which each multiplexer includes first switches providingconnections of multiple panel signal lines to a probe pad, and secondmultiplexers providing connections of common signals to panel signallines.
 16. The display panel of claim 9 which includes a processorconfigured to measure the electrical characteristics of every TFT andevery OLED device in the display panel, and to identify defects andnon-uniformities in the panel based on the measured electricalcharacteristics.